1. Field of the Invention
The present invention relates to a novel silane compound, a method for making it, and an electrophotographic photoreceptor.
2. Description of the Related Art
The charge-transport layer of an electrophotographic photoreceptor generally comprises a binder polymer, such as polyester, and a charge-transport material, which has a low molecular weight, such as a triaryl amine and a tetraaryl benzidine. In order to obtain a film-forming capability, the charge-transport material, which has a low molecular weight, is dispersed in the binder polymer.
However, since the charge-transport material, which has a low molecular weight, is dispersed in the binder polymer, the mechanical strength inherent to the binder polymer decreases, thus presenting a problem that the above-described electrophotographic photoreceptor has an inferior durability.
In order to overcome the problem, there has been proposed a charge-transport polymer which is represented by a polyvinyl carbazole and contains a charge-transport component in the main molecular chain, but such a charge-transport polymer cannot completely solve the problem of electric characteristics and mechanical strength.
Recently, the use of a charge-transport material, which has a low molecular weight, as an organic EL element is drawing attention. In this case, the problem is that it is difficult to obtain a durable and stable element because the material, which has a low molecular weight, is fused by Joule heat generated during use.
The following methods are known as methods for synthesizing conventionally available silane compounds:
(1) A method based on the hydrosilylation of an alkene or alkyne
Various silane compounds having a Si--C bond are synthesized by hydrosilylation of an alkene or alkyne, using a Speier reagent {H.sub.2 PtCl.sub.6 /IPA (isopropyl alcohol)} as described in "Chemistry of Organometallics," Shokabo Publishing Co., Ltd., 1982, pp. 322. This method makes it possible to carry out the synthesis by use of a catalyst in an amount as small as 1.times.10.sup.-7 mol, and therefore it is a very useful method. However, since this method provides by-products, which are caused by isomers or reductive reactions and which need to be strictly removed by a purification treatment such as distillation, this method is not suitable for the synthesis of the present invention using a silane compound having a high boiling point.
(2) A method using a Grignard reaction
As described in "The Chemistry of Organic Silicone Compounds" (Ed. S. Patai, 1989), pp. 655, a silane compound can be synthesized by use of a Grignard reagent. However, since the Grignard reaction is very sensitive to oxygen and moisture and produces a large amount of inorganic salts, whose posttreatment is difficult, the industrialization of this method is very costly.
(3) Synthesis of amine-based silane compound by use of a dehydrochlorination
As described in "The Chemistry of Organic Silicone Compounds" (Ed. S. Patai, 1989), pp. 655, a silane compound can be synthesized by a dehydrochlorination reaction between a silane having a halogen substituent and an amine. The problems relating to this method are that the obtained compounds are limited to compounds of a special type and that the obtained compounds, which necessarily contain an amino group, tend to trap charges and adversely affect the electrical characteristics when these compounds are used as a charge-transport material.
(4) A method using a urethane linkage-forming reaction
Japanese Patent Application Laid-Open (JP-A) No. 3-191,358 proposes to incorporate a silane compound, which has a charge-transport moiety, into an electrophotographic photoreceptor by a urethane linkage. The problem of the compound prepared according to this method is that, since the compound has a hydrogen atom directly linked to a heteroatom, the compound tends to adsorb the moisture in the atmosphere and the organoelectronic device, such as an electrophotographic photoreceptor, which uses this compound, is susceptible to moisture and therefore unstable under environmental conditions.
As to a material, which forms the charge-transport layer of an electrophotographic photoreceptor, two types are widely known: namely, a charge-transport polymer such as polyvinyl carbazole which contains a charge-transport component in the main molecular chain, and a charge-transport dispersion comprising a charge-transport compound of a low molecular weight which is produced by dispersing the charge-transport compound having a low molecular weight such as a triaryl amine, in a binder polymer. Of the foregoing two types, a dispersion comprising a substance having a low molecular weight is now becoming mainstream in the field of electrophotographic photoreceptors, because a variety of substances can be used and because a highly functional charge-transport material can be obtained.
With the advent of highly functional organic photoreceptors, such an organic photoreceptor is now being used in high-speed copiers and printers. However, the level of the function of organic photoreceptors now in use in high-speed copiers and printers is not always satisfactory and there is a strong need for a photoreceptor having a longer service life. One of the important factors which determine the life of the organic photoreceptor is the degree of wear of the surface layer. Since the main stream of the current organic photoreceptors is a so-called laminated photoreceptor, which is produced by laminating a charge-transport layer onto a charge-generation layer, the surface layer is often the charge-transport layer. Even though satisfactory electric characteristics are now being attained, a charge-transport layer based on a dispersion of a substance having a low molecular weight, which is now mainstream in the field of electrophotographic photoreceptor, suffers from weakness in wear due to loss of the mechanical strength inherent in the binder polymer, because a compound having a low molecular weight is dispersed in the binder polymer.
Many attempts have been made to solve the above-mentioned problems, and one of the active studies is the use of a polymeric material as the charge-transport material. For example, U.S. Pat. No. 4,806,443 discloses a polycarbonate obtained by the polymerization of a specific dihydroxyaryl amine with bischloroformate, while U.S. Pat. No. 4,806,444 discloses a polycarbonate obtained by the polymerization of a specific dihydroxyaryl amine with phosgene. U.S. Pat. No. 4,801,517 discloses a polycarbonate obtained by the polymerization of a bishydroxyalkylaryl amine with bischloroformate or phosgene, while U.S. Pat. Nos. 4,937,165 and 4,959,288 disclose a polycarbonate obtained by the polymerization of a specific dihydroxyaryl amine or bishydroxyalkylaryl amine with bischloroformate and a polyester obtained by polymerization of a specific dihydroxyaryl amine or bishydroxyalkylaryl amine with a bisacyl halide. U.S. Pat. No. 5,034,296 discloses a polycarbonate and a polyester obtained from an aryl amine having a specific fluorene skeleton. U.S. Pat. No. 4,983,482 discloses a polyurethane. Japanese Patent Application Publication (JP-B) No. 59-28,903 discloses a polyester comprising a specific bisstyrylbisaryl amine as a main chain. Japanese Patent Application Laid-Open (JP-A) Nos. 61-20,953, 1-134,456, 1-134,457, 1-134,462, 4-133,065, 4-133,066, and others disclose polymers having a charge transporting group, such as hydrazone or a triaryl amine, as a pendant and an electrophotographic photoreceptor which utilizes such a polymer. However, none of the above-described polymeric charge-transport materials is satisfactory in terms of sensitivity, residual potential, and durability as an electrophotographic photoreceptor.
On the other hand, a method has been proposed which comprises dispersing a charge-transport material having a low molecular weight in a binder polymer or in a polymer precursor and thereafter hardening the binder polymer or polymer precursor by means of a curing reaction. For example, JP-A No. 56-48,637 and JP-B No. 56-42,863 disclose an example using an acrylic polymer, while JP-B Nos. 5-47,104, 60-22,347, and 7-120,051 each disclose an example using a silicone polymer or polymer precursor. None of these methods, however, solves the problem because the concentration of the charge-transport material having a low molecular weight needs to be set to such a high value, e.g., 30 to 50%, that the progress of the curing reaction of the binder is inhibited and therefore the charge-transport material having a low molecular weight is separated from between the binder polymers to be worn out.
Further, aiming at the enhancement in the lubricating property of the surface of an electrophotographic photoreceptor, JP-A Nos. 57-5,050, 61-219,049, and 62-205,357 disclose an example wherein a silicon-containing polymer is added, while JP-A Nos. 50-23,231, 61-116,362, 61-204,633, and 61-270,768 disclose an example wherein a fluorine-containing polymer is added. As an attempt to attain the same objective, JP-A No. 63-65,449 discloses an example wherein particles of a silicon-containing polymer are added, while JP-A No. 2-144,550 discloses an example wherein particles of a fluorine-containing polymer are added. None of these methods, however, solves the problem because these polymers or polymer particles have such poor compatibility with charge-transport material or binder polymer that layer separation takes place within the photosensitive layer and the transparency of the layer is lost, thereby deteriorating the electric characteristics of the photoreceptor.
Further, a method has been proposed which comprises adding particles of a tough resin to the photosensitive layer. For example, JP-A No. 60-177,349 disclose an example wherein particles of a melamine resin are added. This method, however, does not solve the problem because the melamine resin particles have such a poor compatibility with the charge-transport material or with the binder polymer that layer separation takes place within the photosensitive layer and the transparency of the layer is lost, thereby deteriorating the electric characteristics of the photoreceptor.
Further, as a method for protecting the surface of an electrophotographic photoreceptor, JP-A Nos. 56-38,055 and 60-55,355 disclose an example wherein an imide-based polymer is used as the surface protecting layer, JP-A Nos. 59-185,347 and 61-217,052 disclose an example wherein a melamine-based polymer is used as the surface protecting layer, JP-A No. 59-46,652 discloses an example wherein an acrylic polymer and a melamine-based polymer are used as the surface protecting layer, "Proc. IS&T 11th Internat. Cong. on Advances in Nonimpact Printing Technol.", pp. 57-59, and JP-A No. 8-15,886 disclose an example wherein a siloxane-based polymer produced by a sol/gel process is used as the surface protecting layer, and JP-A No. 7-333,881 discloses an example wherein an inorganic thin film produced by a plasma CVD process is used as the surface protecting layer. However, many of these surface protective layers have drawbacks, an example of which is that the residual potential increases. Although the method disclosed in "Proc. IS&T 11th Internat. Cong. on Advances in Nonimpact Printing Technol.", pp. 57-59, which uses a sol/gel process, is described as a method advantageous in residual potential and mechanical strength, the concrete structure of the compound is not disclosed at all.
JP-A No. 3-191,358 discloses a method as a type of sol/gel process, wherein one of the materials involved in the sol/gel process is provided with a charge-transport function. Since all compounds disclosed in this case use a urethanelinkage and since the compounds have a hydrogen atom, which is directly linked to a heteroatom and which tends to form a hydrogen bond and therefore is inclined to adsorb moisture in the atmosphere, these compounds are susceptible to the influence of discharge products or humidity in spite of the apparent effect of increasing the mechanical strength of the charge-transport layer. Therefore, it is difficult to obtain a stable image for a long time by use of these compounds because image drift tends to occur when these compounds are used repeatedly or when these compounds are used under conditions of high temperature and high humidity. In addition, JP-A No. 7-72,640 proposes using a monomeric charge-transport compound, which has a carbon-carbon-polymerizable double bond in a triphenyl amine skeleton, singly or in a combination with a polymer so that a polymerization will be effected by means of light or heat to form a tough film. However, since the polymerizable site is only one in the monomer and therefore the density of cross-linkage is remains low after polymerization, a sufficient durability level as an electrophotographic photoreceptor has not been attained.